The invention pertains to an aramid filament yarn provided with a conductive finish, to a method of making such yarn, to the use of said yarn and to a cable comprising said yarn.
A common problem of yarns is breaking of the yarn when used in certain applications. Yarn breaks may occur as a result of over-loading, fatigue, or wear. Ground cable damage may also occur as a result of damage caused by rodents. In elevator cables breakage of cables is a serious safety problem. It is known to add, for example, one or more carbon yarns or metal wire to aramid reinforced elevator cables as a break detector. Such treated, aramid reinforced cables, however, do not have the same mechanical characteristics as cables made of untreated aramid yarns only. Moreover, the breaking characteristics of such carbon yarn or metal wire is different from the breaking characteristics of aramid yarn, thereby hampering accurate indication of breaking danger of the aramid reinforced cable. The other mechanical properties of non-aramid yarns in comparison to the main reinforcing material of the cable complicates the predictability of a cable breakage considerably. It would be an advantage to provide aramid yarn having sufficient conductive properties and nevertheless having the same mechanical characteristics as the untreated aramid yarns in the cable.
Some solutions to the above problems are proposed. In WO 9748832, the yarn was coated with a metal such as nickel and an acid. Such treatment provides metalized yarns, the fiber surfaces of which may be damaged by the acid treatment, leading to a decrease of tenacity and/or elongation properties. In WO 9325748, a process was disclosed for treating aramid fibers with a dispersion of particulate graphite material in a swelling solvent. This method also bears the risk of damaging the fiber surface. Moreover, both methods are very complicated, time consuming, and therefore costly.
Aramid fibers and yarns with an antistatic finish are also known in the art. In U.S. Pat. No. 5,478,648, aramid fibers with a spin finish are disclosed having useful properties for making sheet material. These fibers contain an antistatic finish based on phosphoric and/or phosphonic esters, but are not disclosed to have extremely low specific electric resistance, such as <2.5·104 Ohm·cm. Similarly, U.S. Pat. No. 5,674,615 describes finished aramid fibers for textile sheet materials, which are not reported to have extremely low specific electric resistance. In EP 416,486, finished aramid fibers of relatively high specific electric resistance have been disclosed for use in reinforcing elastomeric and plastic materials. In WO 9215747, aramid fibers have been disclosed which are treated with an antistatic coating. These can be used for reinforcing belts, ropes optical cables, rubber, and composites. The cables herein disclosed are not reported to have extremely low specific electric resistance or to be suitable for use as breaking detector in elevator cables.